This is how the story end but first... About the drivers, I knew that I could skip the automatic reinstall process but I don't have many options here because my laptop model only have one set of newly released drivers for Windows 10 and I don't expect further development on my 2 years old laptop. It's a Lenovo and though it work well on Windows 8.1, I have a feeling that it is not a robust machine. 2 repairs in the first 3 month. Now it seems to be ok but I don't have high hopes for the longevity. About the product key, I could enter a product key but a message told me that it was used by another PC so no activation possible. I've install Windows many time before on this machine without any issue. The product key is in the BIOS and after the installation Windows got activated automatically on the Internet. The same happened with a clean install of Windows 10... silently activated. But when I went back to 8.1, it seems that the product key got stuck to a new machine (which was my machine with Win 10). The end of the story... I called Microsoft and I had to activate my PC through phone. I did a test to see if my product key was reassigned to my Windows 8.1 machine... NO. It seems that each time I will have to activate, I'll have to call for activation. Sad. Windows 10 was not a good idea for me but I'm using it at work and have no problems. Anyway, I won't cry not being able to use it because I'm satisfied with 8.1. Stable and never crash.